Lithography is a process of transferring desired patterns onto a substrate (typically a target area of the substrate) such that patterns are created in different device regions or current regions. Specifically, using exposure in a lithography process, a pattern can be created onto a photoresist layer (made of a photo-sensitive polymer material) disposed on the surface of the substrate to achieve pattern transfer.
With rapid development of semiconductor process technologies, feature sizes of patterns to be exposed by the lithography process have become smaller and smaller, which requires an increasingly higher resolution of lithography. The lithography resolution, or the minimum feature size of lithography is given based on the Rayleigh's criterion, as shown in equation (1):CD=κ1×λ/NA  (1)
wherein λ is the exposure wavelength of the lithography process. NA is the numerical aperture of the projection system of lithographic equipment. κ1 is a lithography process-related factor. CD is the minimum feature size (or critical dimension) of the printed pattern. According to the above equation (1), CD can be reduced by three methods, i.e., reducing the exposure wavelength, increasing the numerical aperture, or decreasing the κ1 factor.
EUV lithography has been considered the most promising lithographic technology. EUV lithography radiation is an electromagnetic radiation having a wavelength ranging from 5 nm to 20 nm and currently is generated by, either laser-produced plasma (LPP), or discharge-produced plasma (DPP).
EUV lithography light source system for generating EUV light usually includes a source-excitation module for generating an EUV-light-producing plasma from a vaporized source material, and a collector module for collecting and collimating the appropriate EUV light generated from the EUV light source-excitation module into an optical non-telecentric system. In a laser-produced plasma system, the source-excitation module usually applies high-energy laser beams to the source material which then produces plasma in the excitation source. In a discharge produced plasma system, high voltage produces plasma which generates EUV light in the excitation source. The collector module has a number of optical elements to direct, select, and collimate the EUV light at a desired wavelength into an output EUV beam.
However, when a conventional EUV light source system excites the solid source material into vapor which then forms EUV-producing plasma, the source material vapor droplets may condense on the EUV light-collecting optical elements. As a result, these condensed droplets can contaminate EUV light reflecting optics in the source system. In addition, the downstream EUV-collecting optics can get contaminated by the flying-over droplets. Once contaminated, light-collecting efficiency goes down quickly. The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.